1. Field of the Invention
The invention relates in general to wireless communication, and in particular, to a method and communication device in a packet-based communication system.
2. Description of the Related Art
Upon receipt of a data packet at a wireless receiving station, the receiving station may detect packet error through Frame Check Sequence (FCS) check, such as parity check coding or signaling check code, after the data packet has been received. If the computed FCS matches the FCS transmitted with the packet, a receiving station may accept the packet, otherwise, the receiving station may discard the packet. In this manner, the receiving station can determine validity of the receipt data.
Due to a variety of factors, such as interference or burst noise, data packets are sometimes corrupted prior of FCS check although the convolutional code has been provided to correct the errors. In general, FCS check is started until the whole packet is fully received and is started after signal processing demodulation has been applied to the received packet. However, such communication flow system would waste tremendous power and have the potential risk to loss the packet.
In the case of hidden node situation, packet loss might occur. For example, station STA A can receive signals from station STA B and access point (AP) while the AP can only receive signal from station STA A (hidden node issues). If STA B sends STA A data packets, STA A may continue receiving the packets from STA B and ignore the signal from AP, resulting in losing data packets from the AP and corrupted data packets from station STA B.
In the case of the packet length information of a data packet is decoded incorrectly, the packet might loss as well. Take a WLAN system for instance, the packet length information in the SIGNAL Field may be decoded incorrectly. This could happen because there is only one parity check bit in the SIGNAL Field. Packet loss may occur if the wrongly decoded packet length is greater than the correct packet length. For instance, if the correct packet length is 1024 bytes and the wrongly decoded packet length is 65535 bytes, the receiver will not stop receiving packet until the packet length reaches 65535 bytes. However, since the correct packet is only 1024 bytes, other stations or access points may transmit packets after they receive 1024 bytes. In this case, data packets are lost due to the wrongly decoded packet length.
Another issue is power consumption; for instance, wireless devices employing portable power storage cells such as batteries, with inherently limited storage capacity, normally would require effective power management solution. FIGS. 1a, b, and c are packet formats compliant with IEEE 802.11n standard, representing non-HT (high throughput), HT mixed mode, and HT Greenfield mode respectively. Non-HT mode is a low throughput data format. HT Greenfield and HT mixed modes are high throughput data formats applicable to MIMO-OFDM (Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing). To guarantee that high throughput (HT) devices, IEEE 802.11n standard has interoperability with legacy devices including IEEE 802.11a/g devices, referred to as Mixed Mode. The SIGNAL FIELD of HT-device in this mode uses a means known as “Spoofing” to reserve the maximum packet length so that the Legacy devices will follow to reserve the maximum packet length for HT-devices. Without termination mechanism, Legacy device will perform signal processing and modulation until the reserve time is reached. However, since this packet is for HT-devices, there is no way that the Legacy devices can decode the packet correctly. In this case, great power consumption is wasted for packets that are expected to be wrong.
Thus a need exists for a communication device and a method to process data packet to reduce power consumption and packet loss.